This invention relates to a method of selecting pieces of scrap silicon containing a prescribed impurity element from a plurality of pieces of scrap silicon containing a variety of impurity elements. It also relates to a method of analyzing scrap silicon to determine the impurity elements contained therein and the contents of the impurity elements.
In order to increase the rate of utilization of silicon, it is desirable to reutilize scrap which is formed during the manufacture of silicon ingots (referred to below as “scrap silicon”). Some scrap silicon contains a large amount of impurities, so it is necessary to increase the purity of scrap silicon before it can be reused.
Semiconductor silicon can be generally divided into n-type semiconductor silicon which uses electrons as mobile charge carriers and p-type semiconductor silicon which uses holes as mobile charge carriers. P-type semiconductor silicon usually contains boron, which has one less valence electron per atom than silicon, as an additive (referred to as a dopant). In contrast, n-type semiconductor silicon usually contains phosphorus (P), arsenic (As), or antimony (Sb), which have one more valence electron per atom than silicon, as an additive.
Accordingly, various types of scrap silicon containing different additives such as boron, arsenic, antimony, and phosphorus in various amounts are formed during the manufacture of silicon products. Since boron forms p-type semiconductor silicon while arsenic, antimony, and phosphorus form n-type semiconductor silicon, semiconductor silicon containing boron as an additive can be easily differentiated from silicon containing arsenic, antimony, or phosphorus as an additive by determining whether a piece of silicon is p-type or n-type using a conventional p-type/n-type checker. If n-type semiconductor silicon can be differentiated from p-type semiconductor silicon, the n-type semiconductor silicon can then be refined in accordance with the type of additive contained therein. From the standpoint of refining, the additives contained in scrap silicon are impurities, so they will be referred to below as impurities or impurity elements.
In the past, in order to refine scrap silicon, n-type scrap silicon was first separated from p-type semiconductor silicon using a p-type/n-type checker, and then the resistivity of individual pieces of scrap silicon was measured. Based on the resistivity, pieces of n-type scrap silicon containing a prescribed impurity element which could be efficiently removed by refining were selected, and the concentration of the impurity element in the pieces after sorting was calculated based on the measured resistivity. The pieces of silicon were then refined under conditions suitable for the calculated concentration. Pieces of silicon containing different types of impurities can to a certain extent be differentiated from each other by measuring resistivity because different types of impurities produce different resistivities. In general, silicon containing arsenic as an impurity has an electrical resistivity of at most 5 milliohm-cm, silicon containing antimony as an impurity has an electrical resistivity from 10 milliohm-cm to 30 milliohm-cm, and silicon containing phosphorus as an impurity has an electrical resistivity of at least 50 milliohm-cm. The relationship between electrical resistivity and the concentration of charge carriers is well known, so if it is assumed that a piece of silicon contains a single impurity, it is easy to calculate the concentration of the impurity from the concentration of carrier electrons.
Recently, however, the above-described impurities have been added in different amounts from in the past so that the ranges of electrical resistivity for different impurity elements may overlap each other. As a result, a given value of electrical resistivity for a piece of scrap silicon may not uniquely identify which impurity element is contained in the piece. Thus, it may be difficult to identify the impurity element in a piece of scrap silicon and to estimate the concentration of the impurity element from a measurement of electrical resistivity alone.
It is conceivable to solve the above-described problem by using an inductively coupled plasma (ICP) emission spectral analyzer to carry out more precise analysis of impurity elements in scrap silicon. However, analysis using an inductively coupled plasma emission spectral analyzer requires a considerable length of time for preparation of specimens prior to analysis. Analysis of impurity elements in silicon can also be carried out by the complete reflection fluorescent x-ray analysis method, but the content of impurity elements in pieces of scrap silicon which do not have a smooth surface cannot be analyzed by this method. Since the content of impurities in scrap silicon is minute, it is not possible to perform analysis by other usual methods of physical analysis.